1. Field of the Invention
The present invention relates to an exposure head for exposing an exposure surface of a photosensitive material or the like with a bundle of light beams modulated by a spatial light modulator in correspondence to image data.
2. Description of the Related Art
Conventionally, various exposure heads for conducting image exposure with light beams modulated in correspondence to image data using a spatial light modulator such as a digital micromirror device (DMD) have been proposed.
As the DMD, a mirror device is used where numerous micromirrors, in which the angles of reflection surfaces thereof change in correspondence to control signals, are two-dimensionally arranged on a semiconductor substrate such as silicon.
The exposure head using this DMD is disposed with, for example, a light source that emits laser beams, a collimator lens system that collimates the laser beams emitted from the light source, a DMD that modulates the laser beams, and an imaging optical system that images, on an exposure plane, the laser beams reflected by the DMD.
In this exposure head, the micromirrors of the DMD are respectively controlled ON-OFF by a control device due to control signals generated in correspondence to the image data, whereby the laser beams are modulated (deflected) to an exposure state or a non-exposure state, and the exposure plane is exposed by the laser beams (the collection of these laser beams will called “beam bundle” hereinafter) modulated to the exposure state.
Here, the imaging optical system is generally configured as a magnification optical system, and the exposure area on the exposure plane is magnified with respect to the area of the effective region of the DMD in which the micromirrors are two-dimensionally arranged. However, when the area of the exposure area on the exposure plane is magnified with respect to the area of the effective region of the DMD by the imaging optical system, the area (spot diameters) of the beam spots at the exposure plane are also magnified in correspondence to the magnification (with respect to the area of the effective region of the DMD) of the area of the exposure area on the exposure plane. Thus, MTF (Modulation Transfer Function) characteristics of the exposure plane deteriorate in correspondence to the magnification of the exposure area.
With respect thereto, there are exposure heads that can solve this problem and have configurations as described in, for example, U.S. Pat. No. 6,133,986 (see FIG. 14) and U.S. Pat. No. 6,473,237 B2 (see FIG. 15).
U.S. Pat. No. 6,133,986 discloses an optical system that combines a double-telecentric projection optical system whose aperture is small but whose image field is large, an array of microlenses that respectively include large apertures and small fields, and a microlens aperture array. In a microlens scanner disposed with this optical system, a printing surface is scanned and exposed with exposure spots formed by the microlens array.
However, in this optical system, there are the problems that a trade-off is necessary between the uniformity of the illumination of the microlens apertures and the suppression of cross-talk of adjacent apertures, and it is difficult to achieve a balance between light use efficiency and obtaining uniform exposure spots at the exposure plane.
FIG. 15 of U.S. Pat. No. 6,473,237 B2 discloses a configuration that uses a group of lenses, a point array such as a microlens array, a grating and an additional group of lenses in order to image, on a subject such as a wafer, pattern information displayed on a pixel panel.
Here, the grating is one for reducing, with a blocking effect, cross-talk light and noise light resulting from the diffraction component of the light illuminating the pixel panel and diffraction and scattering from the pixel panel.
However, when the grating is disposed at a position before the position where the focused light beam resulting from the microlens array is focused, i.e., in the region of so-called Fresnel diffraction, the effect of reducing cross-talk light and scattered light is not sufficient. Also, when the grating is disposed at the focal position of the focused light beams, i.e., the position of so-called Fraunhofer diffraction, the subject cannot be directly placed at the beam focal position because a working distance cannot be secured, and it becomes necessary to image the light on the subject via the additional lens group (imaging lens system).
This imaging lens system has drawbacks in that numerous element lenses become necessary particularly when conducting high-resolution imaging, costs increase, and the imaging lens system requires a large space.